Four screwdrivers works. So does the *handguard tool* which resembles a coathanger, but is heaverier, and usually runs between $10-$20. We had the tapered slip ring on the XM177 *Car 15*, too.
I finally broke down and bought one of those. Got tired of scratching things on my the rifles Mrs. L and I spent a considerable amount of cash on. Shes especially twitchy about her handguard.
https://offhandgear.smugmug.com/Hand-Guard-Gallery/i-BQzTCnS/A
Cant imagine why.....
L